This paper describes clinical and pathologic features of a group of patients with a clinicopathologic syndrome termed "infantile cardiomyopathy with histiocytoid change in cardiac muscle cells". This syndrome is manifested clinically by recurrent cardiac arrhythmias and sudden death, and pathologically by a distinctive type of focal degeneration and histiocyte-like appearance of the muscle cells. This syndrome has been found to occur only in children aged 6 to 24 months, and its etiology is unknown.